1. Field of the Invention
This invention relates to the technical field concerning the production of automatic machines, for example flat-bed or circular knitting machines.
2. Description of the Prior Art
It is known that flat-bed knitting machines generally envisage two needle beds along which a carriage travels longitudinally with alternate motion.
The carriage contains appropriate actuator units for the needles that are fitted inside transverse slits at regular distances along the needle beds.
These actuator units actuate the needles in succession, according to the commands provided by an electronic control unit programmed with a suitable fabric knitting program, so as to achieve the formation of a series of so-called fabric rows.
The machines mentioned require the vertical stretching of the fabric. Various devices are currently used for this purpose, including generally one or more rollers placed below the needles beds and parallel thereto. The roller(s) designed to catch and pull the fabric being formed.
A device of this nature includes, for example, a pair of counter-rotating rollers which are knurled and which are made to touch each other for a preset time interval at the end of each carriage stroke.
Another known device involves a single rotating roller that stretches along the entire length of a needle bed below which it is fitted and which is equipped with a multitude of pins that penetrate between the fabric stitches.
These devices generally produce an uneven stretching in the various machine working areas, giving unsatisfactory results in the knitted fabric. These is in fact a progressive slackening in the fabric, with the passage of each operating unit.
Many of the known devices are extremely complex and this complexity has a negative effect on their performance as well as on their maintenance and production costs.
U.S. Pat. No. 4,854,134, issued to the present applicant, describes a device aimed at making and maintaining the vertical stretching of fabric homogeneous, acting on adjacent sectors of the same fabric.
This device includes a series of coaxial drums with the revolving support of rotating axes and connected to these axes by appropriate elastic parts, each of these drums having a tangential contact with at least two rollers driven by second spring means; the fabric lies between these drums and the relative presser rollers.
Appropriate actuator parts are designed to drive one of the rollers with a rotation of a preset amplitude, to determine the rotation of the relative drum, in phase with the formation of a corresponding portion of the last row of fabric.
Another problem that exits in the traditional stretching devices is the different tension that is often found in different areas of fabric, as shown for example in FIG. 10. The known devices are only able to pull the central part of the fabric, while the areas that extend on either side of this central part are not stretched.